THE CAPILLARY VESSELS. 5 



comparatively slight in structure, but the ventricles are extremely powerful, and contract with 

 great force, by means of a curiously spiral arrangement of the muscular fibres. These latter 

 chambers are used for the purpose of propelling the blood through the body, while the auricles 

 serve to receive the blood from the vessels, and to throw it into the ventricles when they are 

 ready for it. 



By the systematic expansion and contraction of the heart-chambers, the blood is sent on 

 its mission to all parts of the body, through vessels named arteries, gradually diminishing 

 in diameter as they send forth their branches, until they terminate in branchlets scarcely 

 so large as hairs, and which are therefore called "capillaries," from the Latin word capillus, 

 a hair. 



In the capillaries the blood corpuscules would end their course, were they not met and wel- 

 comed by a second set of capillaries. These vessels take up the wearied and weakened globules, 

 carrying them off to the right-hand chambers of the heart, whence they are impelled through 

 a vessel known by the name of the "pulmonary artery," to be refreshed by the air which is 

 supplied to them in the beautiful structure known as the lungs. Meeting there with 

 new vitality — if it may so be called — the blood corpuscules throw off some of their effete 

 portions, and so, brightened and strengthened, are again sent through the arteries from the 

 heart to run their round of existence, and again to be returned to the heart through the 

 veins. 



It is indeed a marvellous system, this constant circular movement, that seems to be in- 

 herent in the universe at large, as well as in the minute forms that inhabit a single orb. The 

 planets roll through their appointed courses in the macrocosmal universe, as the blood globules 

 through the veins of the microcosm, man : each has its individual life, while it is inseparably 

 connected with its fellow-orbs, performing a special and yet a collective work in the vast body 

 to which it belongs ; darkening and brightening in its alternate night and day until it has com- 

 pleted its career. 



In order to prevent other organs from pressing on the heart, and so preventing it from 

 playing freely, a membranous envelope, called from its office the "pericardium," surrounds 

 the heart and guards it. 



The various operations which are simultaneously conducted in our animal frame are so 

 closely connected with each other that it is impossible to describe one of them without trench- 

 ing upon the others. Thus, the system of the circulatory movement, by which the blood passes 

 through the body, is intimately connected with the system of respiration", by which the blood 

 is restored to the vigor needful for its many duties. 



In order to renew the worn-out blood, there must be some mode of carrying off its effete 

 particles, and of supplying the waste with fresh nourishment. For this purpose the air must 

 be brought into connection with the blood without permitting its escape from the vessels in 

 which it is confined. The mode by which this object is attained, in the Mammalia, is briefly 

 as follows : — 



A large tube, appropriately and popularly called the "windpipe," leads from the back of 

 the mouth and nostrils into the interior of the breast. Just as it enters the chest it divides 

 into two large branches, each of which subdivides into innumerable smaller branchlets, thus 

 forming two large masses, or lobes. In these lobes, or lungs, as they are called, the air-bear- 

 ing tubes become exceedingly small, until at last they are but capillaries which convey air 

 instead of blood, each tube terminating in a minute cell. The diameter of these cells is very 

 small, the average being about the hundred and fiftieth of an inch. Among these air-bearing 

 capillaries the blood-bearing capillaries are so intermingled that the air and blood are separated 

 'from each other only by membranes so delicate that the comparatively coarse substance of the 

 blood cannot pass through, although the more ethereal gases can do so. So, by the presence 

 of the air, the blood is renewed in vigor, and returns to its bright florid red, which had been 

 lost in its course through the body, while the useless parts are rejected, and gathered into the 

 air-tubes, from whence they are expelled by the breath. 



The heart is placed between the two lobes of the lungs, and is in a manner embraced by 

 them. The lungs themselves are enclosed in a delicate membrane called the " pleura." These 



